House Democrats have announced a plan that would force the richest 2 million U.S. taxpayers to shoulder much of the cost of an expansion of the nation's healthcare system by imposing a surtax of as much as 5.4% on income above $350,000 a year. The House proposal aims to extend insurance coverage to 37 million Americans over the next decade. Democratic aides said the proposal would cost more than $1.2 trillion over the next 10 years, and would ensure that 97% of Americans were enrolled in a health plan by 2015.